1. Field of the Invention (Technical Field):
The present invention relates to electroslag remelting (“ESR”) electrode immersion depth control systems and methods.
2. Description of Related Art
As shown in FIG. 1, ESR furnaces 10 have been utilized for over 40 years to refine metals and produce fully dense homogeneous ingots 22. The remelting takes place by immersing a consumable metal electrode 14 into a molten slag bath 18 that is resistively heated to a temperature above the melting point of the metal. The electrode gradually melts, forming metal droplets that fall through the slag and collect in a pool 20 under the slag. The molten pool is contained within a water-cooled mold 16 that has a slightly larger size than the electrode. As the electrode melts, it must be translated downward by an electrode drive 12 at a rate related to the fill ratio and the melt rate, as specified by the system controller 24. A complicating factor is that a small amount of slag solidifies on the surface of the mold, changing the amount of metal needed to fill the mold, and changing the thickness of the molten slag on top of the ingot.
To produce a high quality homogeneous ingot with good surface quality, the deviations in the process—specifically immersion depth—need to be minimized. To optimize process efficiency and surface quality, the immersion depth must be maintained at a constant, shallow level. Shallower immersion depths have been shown to result in improved surface quality, hence improved process yields. However, the shallower the immersion depth, the more sensitive the process is to input or external variables, hence, the more difficult it is to control. If the immersion depth is allowed to get too shallow, gaps can form between portions of the electrode surface and the slag, leading to arcing, atmospheric exposure, and deleterious oxidizing reactions. Conversely, too large an immersion depth, or too much variability in depth, can lead to poor surface and metallurgical quality in the ingot.
Again, the ESR process is used to refine metal, remove inclusions, and produce ingots having a uniform solidification grain structure and good surface quality. The immersion depth is an important parameter to control since it has a major effect on the thermal conditions governing melting and solidification. Deviations in immersion depth will alter the thermal environment of the process, inducing changes in the melting process (rate, efficiency, configuration, droplet location and size) and on solidification parameters (rate, direction, molten metal flow). As a result, immersion depth fluctuations will result in changes to the ingot's solidified grain structure, compositional homogeneity, and properties, and affect subsequent processing operations and final product quality.
No system or method currently exists to measure the depth directly so it must be inferred from electrical signals in the process. At present, the ESR immersion depth is controlled in most systems by using the voltage and voltage swing, which is a measure of the variation in the voltage. These methods will be referred to as swing controllers.
The voltage is used because usually it rises as the immersion depth decreases. At a simplified level, the slag can be viewed as a resistor, so the voltage is given by Ohm's Law:V=I[d/(Ak)],where V is the voltage, I is the current, and the resistance of the slag is approximated by the expression in the brackets where d is the distance between the electrode and the molten metal pool, A is the area of the electrode in contact with the slag, and k is the slag conductivity. However, there are numerous simplifications inherent in this treatment, so voltage is only a rough indicator of electrode position. Additionally, the slag thermal environment and chemistry will change over the course of a melt, hence its conductivity is not constant. The amount of molten slag will change during a melt as well, due to slag plating out on the cold crucible walls, further altering the above relationship.
Consequently, while voltage is an effective immediate indicator of relative electrode position in the slag, voltage alone is not adequate to indicate or maintain a constant immersion depth over time, so current controllers use voltage swing as well. Unlike voltage, voltage swing cannot be directly related to the overall system response via an equation such as the one presented above, nor can it be used as an instantaneous indicator of the depth. However, regardless of slag amount, conditions, or properties, the isopotential lines within the molten slag are compressed near the surface of the slag. Consequently, it is believed that voltage variation or swing is less sensitive than voltage to the factors that can change during the course of a melt or between melts or furnaces. As a result, changes in voltage swing can be more reliably, but not quantitatively, related to a changes in immersion depth over the long term.
Existing control methods drive the electrode in response to the error between the system voltage and a voltage set point, with the voltage set point periodically adjusted to match a swing set point. The controllers utilize bi-directional electrode drive to oscillate around the set point, inherently resulting in constant fluctuation of the immersion depth. Because the electrode is moved up as well as down in the slag, existing control systems require a deeper immersion depth for stable operation. By operating at a greater depth as well as introducing increased variation in the depth, surface quality can suffer. A more recently developed ESR control system employs as the electrode drive the combination of a set unidirectional motion and a superimposed periodic fluctuation. This system then superimposes a periodic fluctuation of known amplitude (rather than electrode motion in response to a voltage error) to provide electrode motion relative to the isopotential lines in the slag, and thus generate the voltage swing signal. This system was described in U.S. Pat. No. 5,737,355, to Damkroger, titled “Directly Induced Swing for Closed Loop Control of Electroslag Remelting Furnace”. The drive is described by the equation:Drive Speed=Dave+Di(t),where Dave is the average unidirectional drive speed and Di(t) is the periodic fluctuation. In the long term, positive deviations of voltage swing from the set point are believed to indicate too shallow immersion, and used to increase the basic unidirectional drive speed. Negative deviations are used to do the opposite.
The directly induced swing system eliminated the confounding effect of the system's own drive response on voltage swing. However, it incorporates no short-term response to an error, which limits its ability to operate very near the slag surface. Later modifications of the directly induced swing sought to address this shortcoming by incorporating a (usually limited) voltage error response as was used in the original swing controllers, resulting in a strategy described by the following equation:Drive Speed=Davg+Di(t)+Ke′(Vrms−Vsp),combining an average speed, Davg, an imposed pattern, Di(t) and a speed proportional, Ke′ to the voltage error, i.e., the difference between the measured voltage, Vrms and the voltage set point, Vsp. The average is usually a long term average of the drive speed. Over the long term, the voltage swing is measured and deviations from its set point are used to adjust the voltage set point, usually with a linear gain factor.
Another method of controlling electrode depth is presented by U.S. Pat. No. 6,496,530, titled “Impedance Spike Control of Electrode Depth in Electroslag Remelting”, to Melgaard, et al., which employs a phenomenon known as impedance spikes.
Other ESR control systems known in the art are described in the following references: U.S. Pat. Nos. 4,075,414 and 4,194,078, to Thomas, disclose methods for controlling immersion depth by measuring the slag resistance and moving the electrode and monitoring the resistance change, and by measuring the electromagnetic emission from arcing, respectively. U.S. Pat. No. 4,303,797, to Roberts, discloses determining the drive speed for immersion depth based furnace geometry, voltage, and voltage variation, current and/or current variation. U.S. Pat. No. 4,476,565, to Rashev et al., discloses maintaining depth by detecting arc discharges when the electrode is outside the range. U.S. Pat. No. 4,483,708, to Gfrerer et al., discloses maintaining depth by determining weight of immersed portion of the electrode. U.S. Pat. No. 4,669,087, to Rasheva et al., discloses controlling depth by monitoring arc discharges. U.S. Pat. No. 5,331,661, to Maguire et al., discloses monitoring power phase angle for depth control. U.S. Pat. No. 5,568,506, to Schlienger, discloses using a constant voltage power supply and constant drive speed to hold the depth.
While some of the above mentioned inventions were able to reduce the immersion depth fluctuations over the standard swing controllers, none provided any means for estimating the actual immersion depth. The present invention allows an ESR furnace to be controlled in a more stable manner, by characterizing the actual immersion depth as a function of impedance and incorporating this into the electrode positioning control strategy.